Ultrasonic systems usually consist of a sound head that sends signals into the body and receives the echoes as well as a system that processes the received echoes into images. A sound head usually contains a matrix of ultrasonic signal transmitters that transmit the ultrasonic energy pulses into the body region to be investigated and receives reflected ultrasonic energy pulses from the region to be investigated. The signal transmitters (comparable to microphones) convert the received ultrasonic energy pulses into weak electric signals which pass over a cable into the processing unit. The incoming signals of the individual signal transmitters are combined by so-called beam forming. The processing unit generates an image of the body region investigated by means of signal and image processing operations. Matrices are used for dynamic focusing for the image construction and to improve the signal-to-noise ratio. The latter is a very important factor in the design of the overall system.
Conventional ultrasonic heads consist of matrices of piezoelectric signal transmitters which are connected by high-quality micro-coaxial cables to the processing unit. To obtain a high-quality image, a multitude of signal transmitters are needed. A higher number of signal transmitters also means that the complexity of the ultrasonic head is dramatically increased for the following reasons:
The acoustic impedance (characteristic wave impedance) of the piezoelectric signal transmitters must be adapted to that of the body tissue. This is accomplished by using various silicone rubber layers, the thickness of each amounting to ¼ of the wavelength.
Emitted signals are 100 dB “louder” than the received signals. Accordingly, very wide control ranges are necessary.
Since the interval of time between the transmitted signals and the reflected signals amounts to only a few microseconds, a complex attenuation is required to achieve a high axial resolution, to prevent noise after transmission and to shorten the pulse.
The individual signal transmitters in the matrix must be insulated from one another to prevent both acoustic and electric interference. This is an enormous expense in design and production not only of the signal transmitter matrix but also in the cable and in the interface to the processing unit. Accordingly, only a limited number of signal transmitters can be used, which keeps the image quality at a low level.
The electronic signals generated by the piezoelectric signal transmitters are on the order of magnitude of a few microvolts. Therefore, only extremely high-quality micro-coaxial cables can be used to prevent crosstalk between the channels. Due to the rapid reduction in signal strength, the cables also cannot be very long, which limits their usability in everyday clinical practice.
Although it is true of image quality that more signal transmitters are advantageous, this is not the case with regard to the cost of the system and user convenience, as described above.
The electronics of a conventional ultrasonic system are extremely complex. The systems must meet high requirements with regard to additional control ranges, high-frequency analog signals that must be digitized, and data processing of a few dozen gigabits per second.
All the conventional systems have an analog module which has various channels for the transmission and reception of the signals. Each channel receives an analog signal, processes it and converts it to a digital signal. In the case of transmission, this signal processing takes place in the opposite order. The more channels a system has, the better the resolution, the signal-to-noise ratio and the control range. In the reception mode, noise suppression is very important because the lowest signals are only a few nanovolts strong, which corresponds to the level of the noise. Even with very expensive high-quality components and the newest circuit board designs, the control range of the system is reduced by on the order of magnitude of 20 dB due to noise. This is a very critical order of magnitude, which is very important for low B-mode images and Doppler flow measurements.
High-end systems today use up to 256 channels to solve precisely this problem. The disadvantage of this procedure is an explosion of system costs, power consumption and size. Each channel increases the cost of materials, increases the size of the circuit boards and requires additional power. Furthermore, more channels increase the complexity of the overall electronic system, which drastically increases development costs. There are potential improvements through analog ASICs (Application-Specific Integrated Circuits). Due to the small number of systems sold—approximately 30,000 to 40,000 ultrasonic devices are sold per year throughout the world—this approach is extremely inefficient.
Against this background, the object of the present invention was to provide a method for measuring ultrasonic waves and a corresponding ultrasonic probe which would eliminate the disadvantages known from the state of the art as described above.